This specification relates to horn type loudspeakers. Horns are structures that affect the directional characteristics of acoustic energy that is radiated into the horn by an acoustic driver. Typically, the horn causes the acoustic energy to be radiated in a directional pattern, which will be discussed later. Horns typically include a throat, into which acoustic energy is radiated, and a mouth, from which acoustic energy is radiated to the environment. It is desirable that the impedance of the horn at the mouth match the impedance of the free air. If there is an impedance mismatch, some of the acoustic energy may be reflected back into the horn which creates audible resonances. One technique used to dampen the resonances is to create resistive leaks in the horn walls.
For example, FIG. 1 of U.S. Pat. No. 3,174,578 (hereinafter '578) is reproduced as FIG. 1 of the attached drawings. According to '578, the horn 1 of FIG. 1 has innumerable pores or slits in the wall 2, which pores are arranged near the mouth portion as described hereinafter. Air is allowed to leak through the pores or slits and is damped by acoustical resistance 3 such as silk cloth or wire gauze stretched over the pores or slits.
Further according to '578, the sound wave travelling from the horn throat 4 is initially a plane wave 5 and gradually changes to a spherical wave 6 as it passes the region where the wall has pores or slits. The change in shape of the sound wave results from the fact that the horn tends to have the character of the open end of an acoustical tube at the portion of the horn having pores owing to the leakage through the wall and the wave at the open end assumes the form of spherical wave as is well known.
Still further according to '578, upon reaching the horn mouth 7 the sound wave surface can take the form of a sphere 8 whose radius is almost equal to that of the horn mouth if suitable leakage through the wall is allowed. If the radius of the sound wave at 8 is equal to the radius of the horn mouth 7, the sound wave does not reflect at the horn mouth and the output sound pressure of the horn has no peak nor [sic or] dip at varying frequencies.
Another example is described in U.K. Pat. 22,965, of which FIG. 1 is reproduced as FIG. 2. According to U.K. Pat. 22,965, FIG. 2 shews [sic] the base of a trumpet. This base is perforated with holes of various diameters arranged parallel with the rim• of the trumpet and distributed uniformly over the base. Without entering into the theory of these perforations, it can nevertheless be stated that good results are obtained by arranging the holes along the generating lines of the trumpet, by diminishing the diameter of the holes as they recede from the base, by making the diameter of the holes in the first row proportionate to the diameter of the trumpet, and by stopping the perforations at a certain distance from the base.
Still another example is described in U.S. Pat. No. 1,840,992 (hereinafter '992), of which FIG. 1 is reproduced below as FIG. 3. According to U.S. Pat. No. '992, carrying out [the] invention, the horn is made of a body 5 which may be made of any suitable material of sufficient stiffness to maintain the horn in its proper shape. [The inventor] contemplate[s] the use of thin perforate metal, papier-mache, wire mesh, wood, wicker, bamboo, or any other material suitable for the purpose. The stiff material 5 forming the body of the horn is preferably provided throughout its entire surface with a series of perforations 6. Any foraminous material such as a skeleton frame work of strip material with spaces between will suffice. These perforations can be very closely spaced; they may vary in shape and size; they may be situated in various areas of the horn and they may be in the form or slits, slots or various shaped openings. These perforations may be placed throughout the area of the horn or they may be localized to suit different requirements of sound distribution. It is also entirely feasible to use one or more large openings in place of a series of small openings, if desired.
Further according to '992, to dampen out undesirable resonance in the horn, [The inventor] find[s] it desirable to line both inner and outer faces of the horn with a suitable sound dampening material such as indicated at 7 and 8. This material may be in the form of loosely compacted felt, burlap, carpet, plush, felt, sponge rubber or some other material of similar characteristics placed on in single or multiple thickness [sic thicknesses]. It is not perforated but is preferably of “open” characteristics to the extent of permitting penetrability of the sound.
Still further according to '992, through the structure just described, it will be seen that by the provision of openings in the side walls of a sound reproducing device, the directional characteristics of such device are dissipated. There is a tendency for the sound to extend laterally through the openings and dampening covering, the amount of such lateral disposition of the sound being dependent upon the size and shape of such openings and upon the location of the same, together with the thickness and density of the fabric covering.